1. Field of the Invention
The present invention relates to a tubular object manufacturing method, and more particularly to a tubular object manufacturing method in which liquid heat-resistant resin is applied onto the surface of a core member such as a columnar mold, cylindrical mold, or the like, and this applied liquid heat-resistant resin is solidified or hardened. Moreover, the present invention relates to tubular objects obtained in such a manner.
2. Description of the Related Art
Heat-resistant films are used in various applications such as flexible print substrates, insulators for electrical equipment, magnetic tapes, and the like. Among these heat-resistant films, tubularly molded heat-resistant films are used for fixing belts, etc. in electronic photocopiers, facsimile machines, printers, etc. That is, with respect to electronic photocopiers etc., there is a method for thermally fusing the toner on the recording paper and fixing the toner image on the recording paper, in which the toner is directly heated on the recording paper by a heater via a film-like endless fixing belt (tubular object). With respect to this fixing belt, because superior heat resistance, strength, Young's modulus of elasticity, etc. are required, a polyimide film with superior heat resistance and mechanical strength is generally used.
As the manufacturing method for tubular objects including polyimide films, a method has been proposed (Unexamined Japanese Patent Publication (kokai) Nos. Hei-7-164456 and Sho-62-19437) in which a polyimide precursor solution is applied by the dipping method onto the outer circumferential surface of a core member such as a columnar mold or to the inner circumferential surface of a cylindrical mold; next, the excess resin is shaved off by dead-weight dropping of a die-shaped object (in the case of coating onto the outer circumferential surface) or a bullet-shaped object (in the case of coating onto the inner circumferential surface) from the upper end of the application area; after obtaining the predetermined film thickness, the coating layer is heated to form a hardened coating of polyimide.
In the conventional tubular object manufacturing method, improvements have been desired particularly in the following two points. The first point is that, because the viscosity of the not-yet-hardened heat-resistant resin is reduced by heating after formation of the coating layer, there have occurred cases where sagging occurs in the coating layer, depending on the viscosity of the heat-resistant resin or the heating condition, to thereby generate irregularity in thickness of the coating layer. The second point is that several centimeters at the upper end of the coating layer cannot be used for the product because the upper end of the coating layer may become slanted at the time of dipping, or because when starting the dropping of the die-shaped or bullet-shaped object, the circumferential position of these dropping objects is unstable. Furthermore, in order to facilitate removal of the mold, it is necessary to remove approximately 1 cm of resin at the lower end part of the mold after application has been conducted to the lower end by dipping. That is, there are portions which cannot be used at the opposite ends, resulting in resin loss. In addition, with the conventional tubular object manufacturing method, it has been impossible to manufacture, for example, crown-shaped or drum-shaped tubular objects which have thicknesses varying in the axial direction, or tubular objects which have circumferential strength larger than axial strength.
The aforementioned tubular object manufacturing method is not only applicable to polyimide, but also to any other liquid heat-resistant resins, for example, unhardened liquid thermosetting resin, solutions of unhardened thermosetting resin, or solutions of thermoplastic resin. However, the cases where tubular objects are manufactured using the other liquid heat-resistant resins, the same problems of thickness irregularity and resin loss occur as in the case of using polyimide. And there is the same problem that it is impossible to manufacture tubular objects having thicknesses varying in the axial direction or tubular objects with circumferential strength larger than axial strength.